1. Field of the Invention
One of the aspects of the present invention relates to an image forming apparatus such as a copying machine, a laser beam printer and the like, and for example, to an image forming apparatus including an endless belt, a heater in contact with an inner surface of the endless belt, and a nip forming element that forms a fixing nip together with the heater via the endless belt.
2. Description of the Related Art
When an image forming apparatus is originally designed for use in an area where a commercial power supply has a voltage in a 100-V range (for example, 100 V to 127 V), if this image forming apparatus is used in an area where a commercial power supply with a voltage in a 200-V range (for example, 200 V to 240 V) is supplied, maximum electric power available to a heater of a fixing unit increases by a factor of 4. The increase in maximum available power to the heater can cause a significant increase in a high-frequency current or flicker generated during a process of controlling power of the heater by means of a phase control, a wavenumber control, etc. Besides, if thermal runaway occurs in the fixing unit, electric power associated with the thermal runaway is 4 times greater, and thus circuits used need to be capable of quickly responding. Therefore, the most common way to allow a single image forming apparatus to be used in both 100-V and 200-V power supply areas is to select a heater with a proper resistance depending on the area and install the selected heater.
A technique has been proposed to realize an apparatus for universal use in both 100-V and 200-V commercial power supply areas by switching the resistance of the heater using a relay or other switching devices. More specifically, for example, Japanese Patent Laid-Open No. 7-199702 discloses an apparatus in which first and second resistance heating elements are formed on a heater substrate, and the apparatus is adapted to be capable of switching between a first operation mode in which the first and second resistance heating elements are connected in series and a second operation mode in which the first and second resistance heating elements are connected in parallel whereby it is possible to switch the resistance of the heater depending on the commercial power supply voltage such that the apparatus can be used regardless of where the commercial power supply voltage is 100 V or 200 V. In the technique in which the first and second resistance heating elements are connected in series or in parallel depending on the commercial power supply voltage, it is possible to switch the resistance of the heater without changing the heating area of the heater. In other words, the two resistance heating elements generate heat regardless of whether the apparatus is used in the 100-V area or 200-V area, and thus a fixing nip has a constant temperature distribution in a recording sheet conveying direction regardless of the area in which the apparatus is used. As a result, the performance of fixing toner images does not depend on the area in which the apparatus is used.
However, in this technique, if a failure occurs in a relay for switching the resistance of the heater, a situation can occur in which electric power is supplied only to one of the two resistance heating elements. Hereinafter, such a state will be referred to as a partially powered state. The partially powered state can produce a problem such as a reduction in durability life of the fixing unit or degradation in the performance of fixing compared with that in the normal state. Thus, it is necessary to detect whether the apparatus is in the partially powered state.